Hindered hydroperoxide complexes

A wide variety of solvents has been used for epoxidations, but hydrocarbons are generally the solvent of choice 428 Recently, it has been shown434 that the highest rates and selectivities obtain in polar, noncoordinating solvents, such as polychlorinated hydrocarbons. Rates and selectivities were slightly lower in hydrocarbons and very poor in coordinating solvents, such as alcohols and ethers. The latter readily form complexes with the catalyst and hinder both the formation of the catalyst-hydroperoxide complex and its subsequent reaction with the olefin. 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Secondly, the interaction of hindered amines with hydroperoxides was examined. At room temperature, using different monofunctional model hydroperoxides, a direct hydroperoxide decomposition by TMP derivatives was not seen. On the other hand, a marked inhibitory effect of certain hindered amines on the formation of hydroperoxides in the induced photooxidation of hydrocarbons was observed. Additional spectroscopic and analytical evidence is given for complex formation between TMP derivatives and tert.-butyl hydroperoxide. From these results, a possible mechanism for the reaction between hindered amines and the oxidizing species was proposed. 

Complex formation constants could also be determined directly from UV spectrophotometric measurements. Addition of tert.-butyl hydroperoxide to a solution of nitroxide I in heptane at RT causes a shift of the characteristic absorption band of NO at 460 nm to lower wavelengths (Fig. 9). This displacement allows calculation of a complex equilibrium constant of 5 1 1/Mol. Addition of amine II to the same solution causes reverse shift of theC NO" absorption band. From this one can estimate a complex formation constant for amine II and +00H of 12 5 1/Mol (23 2 1/Mol was obtained for tert.-butyl hydroperoxide and 2,2,6,6-tetramethylpipe-ridine in ref. 64b). Further confirmation for an interaction between hindered amines and hydroperoxides is supplied by NMR measurements. Figure 10a shows part of the +00H spectrum in toluene-dg (concentration 0.2 Mol/1) with the signal for the hydroperoxy proton at 6.7 ppm. Addition of as little as 0.002 Mol/1 of tetra-methylpiperidine to the same solution results in a displacement and marked broadening of the band (Fig. 10b). 

In some cases (e.g., gasoline), autoxidation of hydrocarbons is undesirable, and trace amounts of metal catalysts may often be deactivated by the addition of suitable chelating agents. The latter affect the catalytic activity of metal complexes by hindering or preventing the formation of catalyst-hydroperoxide or catalyst-substrate complexes by blocking sites of attack or by altering the redox potential of the metal ion. 

The synthetic value of the reaction lies in the modification of these organoboranes. The commonest reaction involves the decomposition of the borane by alkaline hydrogen peroxide. The highly nucleophilic hydroperoxide anion attacks the electron-deficient boron with the formation of an ate complex. Rearrangement of this leads to the formation of a borate ester which then undergoes hydrolysis to an alcohol in which an oxygen atom has replaced the boron (Scheme 3.15). The overall outcome of this reaction is the anti-Markownikoff hydration of the double bond. The regiochemistry is the reverse of the acid-catalysed hydration of an alkene. The overall addition of water takes place in a cis manner on the less-hindered face of the double bond. 

U626 is a rather complex molecule. Very interestingly the phosphite moieties in U626 can have also an important role in the stabilization. Phosphites are well-known stabilizers and they are called secondary antioxidants, while hindered phenol-based stabilizers are known as primary antioxidants. Phosphites have the ability to react with hydroperoxides to yield phosphates according to scheme 5.[20,38] U626 combines primary and secondary stabilizers in the same molecule. 

For example, the ferf-amyl hydroperoxide-molybdenum complex is presumed to attack a-pinene selectively from the least hindered side to give only one isomeric epoxide [388], equation (240). 

Keywords sunlight, heat, hydroperoxides, autoxidation, weathering, photochemical degradation, processing, phosphites, sulphur compounds, metal thiolates, nickel complexes, UV absorbers, HALS, metal deactivators, hindered phenols. 

The above cyclic mechanism has been carefully scrutinized by many workers in the past few years, and, in fact, it has been concluded that it alone cannot fully account for the high photoprotective efficiency of the parent amine molecule. The nitroxyl radical itself is a radical scavenger but is not as effective as hindered phenols in competing with oxygen for alkyl radicals. To account for this deficiency in the cyclic mechanism it has been suggested, and indeed confirmed, by many workers, that hindered piperidine stabilizers and their derived nitroxyl radicals form weakly bonded localized complexes with hydroperoxides in the polymer (Scheme 8). This mechanism raises the local concentration of nitroxyl radicals in regions where alkyl radicals are generated after the... 

Organic phosphorous(III) compounds are very effective hydroperoxide decomposers. They react stoichiometrically with hydroperoxides, oxidizing the phosphite to phosphate. Sterically hindered aromatic phosphites can also react as chain scission antioxidants. Since phosphites and phosphonites are sensitive to hydrolysis, hydrolysis resistant derivates have to be used. Their contribution to long-term stabilization is small, because they do not react directly with oxygen. In addition to the reaction of phosphites with peroxides and oxygen radicals, positive effects are also described due to complexing with catalytic metal residues. 

Transition metals will promote oxidative reactions by hydrogen abstraction and by hydroperoxide decomposition reactions that lead to the formation of free radicals. Prooxidative metal reactivity is inhibited by chelators. Chelators that exhibit antioxidative properties inhibit metal-catalyzed reactions by one or more of the following mechanims prevention of metal redox cycling occupation of all metal coordination sites thus inhibiting transfer of electrons formation of insoluble metal complexes stearic hinderance of interactions between metals and oxidizable substrates (e.g., peroxides). The prooxidative/antioxidative properties of a chelator can often be dependent on both metal and chelator concentrations. For instance, ethylene diamine tetraacetic acid (EDTA) can be prooxidative when EDTAiiron ratios are <1 and antioxidative when EDTAiiron is >1. The prooxidant activity of some metal-chelator complexes is due to the ability of the chelator to increase metal solubility and/or increase the ease by which the metal can redox cycle. 

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